This invention relates to a process for the production of columnarly solidified bodies by melting metals and subsequently solidifying them in a crucible wherein the dissipation of heat is greater at the upper or lower end of the crucible than at the sides.
In view of the shortage and increasing cost of fossil fuels, the search for renewable energy sources has been stepped up worldwide. In this connection, particular efforts are being made to find ways of utilizing energy from the sun.
In addition to thermal energy recovery techniques, the direct conversion of solar energy into electricity using the photovoltaic effect in suitable semiconductors is a very promising alternative method of meeting the future energy demand.
Solar cells based on silicon are now virtually the sole power supply basis for space satellites. However, the silicon used from semiconductor technology is generally too expensive for terrestrial energy production.
A major step in the cost reduction chain is the replacement of complicated techniques for growing single crystals by a rapid and economical crystallization process.
Over recent years, there has been no shortage of attempts to develop inexpensive processes for producing silicon suitable for solar cells.
For example, U.S. Pat. No. 4,243,471 describes a process for producing crystalline silicon by directional solidification. In this process, the expansion of around 10% which the silicon undergoes in changing from the liquid to the solid state is counteracted by allowing silicon which has been melted in a medium-frequency-heated graphite susceptor to solidify from the bottom by gradual lowering of the crucible (Bridgman technique). The use of an ingot mold having an expansion coefficient of 3.0 to 4.3 10.sup.-60 C.sup.-1 is intended to ensure that directionally solidified silicon is not exposed at these temperatures to any thermal stresses arising out of adhesion of the silicon to the vessel wall or out of the greater coefficient of expansion by comparison with the silicon. Although this process does avoid the onset of crystallization on the crucible wall at considerable expense, it is attended by the serious disadvantage that it requires an elaborate and hence expensive apparatus so that an increase in production by operating several such units in parallel does not lead to the desired reduction in costs.
According to German 3,138,227 (=U.S. Ser. No. 191,260) or U.S. Pat. No. 4,218,418 silicon is fused in a quartz vessel and is held at a temperature just above its melting point. A monocrystalline silicon seed arranged from the outset on the bottom of the quartz vessel and kept by cooling at a temperature which prevents it from melting serves as a seed crystal in this process. The effect of this is that crystallization takes place from the bottom over a period of several hours by cooling of the silicon with development of a convex phase boundary. Using this process, it is possible to obtain predominantly monocrystalline zones, in addition to which the silicon melt is additionally purified because of the different solubilities of most of the impurities in liquid and solid silicon. The disadvantage of this so-called heat-exchange process lies in the slow solidification of the melt volume because the entire heat of fusion (which for silicon is large) has to be dissipated through the gas-cooled monocrystalline silicon plate and the deficient heat capacity of the cooling gas. Even with relatively small blocks, this leads to long crystallization times of several hours. Silicon blocks measuring 20.times.20.times.10 cm.sup.3 require crystallization times of several days.
The purifying effect accompanying the slow crystallization in view of the segregation coefficients for impurities in silicon is common to all processes in which the crystallization rate is directional and adjustable under control.
According to German 2,745,247, shaped silicon bodies having a columnar structure are obtained by pouring a silicon melt into a mold and allowing the melt to solidify, the contact surface of the mold with one of the two largest mutually opposite boundary surfaces of the melt being kept at a maximum temperature of 1200.degree. C. and the opposite boundary surface of the melt being exposed to a temperature some 200.degree. to 1000.degree. C. above that temperature, but below the solidus temperature of silicon. The mold is then cooled, the heat of fusion of the silicon being dissipated by intensive cooling of the mold base.
Additional losses of heat through the walls of the mold cannot be avoided in this way. Accordingly, the heat of fusion is not dissipated through the bottom of the mold alone, but also through the side walls.
In view of the concave form of the phase boundary, mechanical separation of the zones rich in impurity atoms from those of low concentration is only possible with heavy losses of material.
German 3,223,896 describes a process and an apparatus for the directional solidification of melts in which heat of crystallization is dissipated as in German 3,138,227 through the bottom of the mold without using a seed crystal. Several heating systems and a cooling system are required for carrying out this process, making it expensive both in terms of cost and in terms of equipment.
Accordingly, the object of the present invention is to provide a simple and economic process with which relatively large amounts of silicon can be melted and which does not have any of the disadvantages of the described processes.